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   -创建 Cookie:
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   -读取 Cookie:
   -var x = document.cookie;
   -修改 Cookie (覆盖旧的):
   -document.cookie="username=John Smith; expires=Thu, 18 Dec 2043 12:00:00 GMT; path=/";
   -删除 Cookie (设过期时间):
   -document.cookie = "username=; expires=Thu, 01 Jan 1970 00:00:00 GMT";
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<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title>菜鸟教程(runoob.com)</title>
	</head>
	<head>
		<script>
			<!--设置 cookie 值的函数-->
			function setCookie(cname,cvalue,exdays){
				var d = new Date();
				d.setTime(d.getTime()+(exdays*24*60*60*1000));
				var expires = "expires="+d.toGMTString();
				document.cookie = cname+"="+cvalue+"; "+expires;
			}
			<!--获取 cookie 值的函数-->
			function getCookie(cname){
				var name = cname + "=";
				var ca = document.cookie.split(';');
				for(var i=0; i<ca.length; i++) {
					var c = ca[i].trim();
					if (c.indexOf(name)==0) { return c.substring(name.length,c.length); }
				}
				return "";
			}
			<!--检测 cookie 值的函数-->
			function checkCookie(){
				var user=getCookie("username");
				if (user!=""){
					alert("欢迎 " + user + " 再次访问");
				}
				else {
					user = prompt("请输入你的名字:","");
					if (user!="" && user!=null){
						setCookie("username",user,30);
					}
				}
			}
		</script>
	</head>

	<body onload="checkCookie()">
	</body>

</html>
